With apparatus in the field of entertainment electronics, such as for example radio receivers, cassette recorders, CD players and the like, it is usual to provide in the audio signal path adjustable filter circuits through which the user can take influence on certain sound characteristics, such as for example treble boost and treble cut or bass emphasis and bass cut. The adjustment of suitable filter characteristics is made by the user by means of operating members provided therefor. The adjustable filter circuits, for purposes of influencing the audio signals in entertainment electronics equipment, are designed as a rule such that they permit in particular also a neutral setting in which they have as little influence as possible on the audio signals with respect to frequency and phase response. In case the user desires no emphasis or de-emphasis at all of certain frequency ranges of the audio signal, he will select this neutral setting of the respective filter circuit.
With respect to numerous apparatus to be associated with the field of entertainment electronics, such as with compact radio receivers used in motor vehicles, a trend can be observed for a long time to achieve a cost reduction in manufacture by putting together the entire electrical circuit of the receiver from as few highly integrated electronic components as possible.
FIG. 6 shows a block diagram of an exemplary audio receiver. Radio frequency signals delivered from a radio frequency antenna 10 are supplied to a radio frequency receiver, hereinafter referred to as "radio frequency processor" RFP. Radio frequency processor RFP comprises radio frequency pre-stages, tuners, intermediate frequency amplifiers and intermediate filters as well as demodulators. A low frequency signal constituting the useful signal of the station received is fed to an audio signal processor ASP. This low frequency signal does not only include the demodulated audio signal, but also auxiliary signals for stereo reception, ARI identification, RDS and so on.
The audio signal processor ASP in particular converts the low frequency signal delivered by radio frequency processor RFP into audio signals which are fed to an audio power processor APP. The audio power processor APP comprises in particular a power amplifier 12 amplifying the output signals delivered from audio signal processor ASP, to such an extent that these can be reproduced through loudspeakers 13. Furthermore, the audio power processor APP comprises a voltage regulating module 14 for power supply.
The audio signal processor ASP may be fed, via additional analog inputs, with other audio signals, for example from cassette recorders, CD players or the like.
The audio receiver described is controlled in its entirety by a microcontroller 15 which is provided with an operating unit 20 that is accessible to the user. The exchange of data between microcontroller 15 on the one hand and radio frequency processor RFP, audio signal processor ASP and audio power processor APP on the other hand takes place through a conventional I.sup.2 C bus.
FIG. 7 shows a schematic block diagram of the audio signal processor ASP of the audio receiver shown in FIG. 6. The audio signal processor ASP has a number of analog audio signal inputs 50a to 50j that are connected to corresponding input lines of an analog multiplexer 55. A stereo output signal of analog multiplexer 55 is fed via output lines 57a and 57b to a chain of series-connected blocks 60, 62, 64, 66, 68 acting as signal processing means.
The audio signals first enter a muting filter ("mute") 60, and then in succession a volume control with loudness filter 62, a second muting arrangement with soft muting property ("soft mute") 64, a bass control ("bass") 66 as well as a treble control 68 ("treble"). The stereo audio signals then are fed to a number of audio driver amplifiers 70, where they are conditioned to such an extent that they are suitable for controlling the power output stages in power amplifier 12 within audio power processor APP.
In this respect, each channel of the stereo signal is split to a first sub-channel for a first loudspeaker to be installed in the front of the passenger compartment and a second sub-channel for a second loudspeaker to be installed in the rear of the passenger compartment.
The audio signal processor ASP comprises furthermore a number of usual functional blocks 72 to 82 which are necessary for processing the stereo differential signal, the ARI signal etc. Finally, there is provided a pause circuit 90 and a power supply means 95.
It is apparent that the integrated audio signal processor ASP represented in FIGS. 6 and 7 does not constitute the sole possible embodiment of such a processor. In particular, the audio signal processor may also be implemented without the functional units for processing FM stereo signals. The type and the number of the LF stages for taking influence on the audio signal may also vary for each particular case.
The circuit described hereinbefore involves the disadvantage that there is no complete integration of the audio filters 62, 66, 68 since numerous RC members with high resistances and/or capacitances are realized by means of external components because of a too large space requirement of the resistors R and capacitors C, respectively.
The document GB-A-2,138,231 discloses a signal transmission circuit with MOS switches. This circuit comprises capacitors that are switchable by said MOS switches, as well as an operational amplifier. Switching of the MOS switches takes place under the control of the output signal of a voltage-controlled oscillator whose output frequency can be varied by means of a variable voltage source, whereby the transmission characteristics of the known signal transmission circuit is adjustable. In another circuit, also known from said document GB-A-2,138,231, the feedback path of the operational amplifier has disposed therein a parallel connection including a feedback resistor and an SC filter. In a first mode of operation, the SC filter is located in the feedback loop, whereas in a second operational state it is not active in the feedback loop.
The document U.S. Pat. No. 4,453,143 discloses an equalizer having a filter and a summing amplifier. Both the summing amplifier and the filter contain switched capacitors. The summing amplifier has an operational amplifier having, however, no filters at all provided in its feedback loop.